The present invention refers to a polyamide moulding composition having an excellent adhesion to metal surfaces. Moreover, the present invention refers to structural parts with a metal component directly bonded to a thermoplastic component (interconnected without using a bonding agent/layer). The present invention also relates to a method for manufacturing of such structural parts by an injection moulding or extrusion processes.
Metal/plastic composite parts combine the respective positive properties of the components metal and plastics in one part. Parts of two different components, metal and plastics, are here referred to as “hybrid” parts. The properties and working behavior of metals and plastics, however, greatly vary and can therefore not be easily interconnected such that a permanent and loadable connection is obtained.
From EP 0 370 342 A2 a composite part is known in which reinforcing ribs of plastics are injected to a basic body of metal. The metal basic body comprises openings through which the plastics are injected. This means that this is a positive connection where the plastics are anchored in the metal. Alternatively a positive connection can be achieved by bead-locking of the metal component. Such positive connections are not satisfactory for loaded parts with respect to their bond strength. Moreover, the parts are corrodible as moisture can penetrate between the metal component and the plastic component due to capillary action.
In order to improve the adhesion between a metal surface and a polymer DE 10 2014 008 815 A1 proposes the roughening of the metal surface by pulse laser irradiation. The microscopic undercuts generated in this way can be partially filled by a polymer during the following injection moulding process. But the pretreatment of the metal is time-consuming and only low molecular weight polymers are able to successfully infiltrate the microscopic undercuts.
Because only a few polymers are suitable for direct bonding to unmodified metal surfaces WO 2008/089873 A2 proposes to introduce an adhesion layer between polymer and metal layer. After the adhesion layer is applied to the metal surface the polymer layer is heat-fusion-bonded to the adhesion layer forming a three-layer composite.
U.S. Pat. No. 4,690,856 describes laminates consisting of a metal layer and a layer made from a polyamide adhesive composition having high peel strength. The adhesive composition comprises a polyamide, a modified polyolefin and an aminosilane compound and is provided as sheet. The polyamide-polyolefin core layer is heat-fusion-bonded to the metal layer by press forming or continuous forming using a roll.
DE 10 2014 004 430 A1 refers to a method for the lamination of a metal surface with a self-adherent polymer comprising a functional group selected from a acid or silane compound by injection moulding or extrusion processes.
WO 2015/159834 A1 refers to a semi-aromatic polyamide resin composition which contains a semi-aromatic polyamide and a polyhydric alcohol, and which is characterized in that the mass ratio of the semi-aromatic polyamide to the polyhydric alcohol is from 99.95/0.05 to 90/10 and that the semi-aromatic polyamide has a melting point of 300-350° C. The addition of polyhydric alcohol improves the heat and hydrolytic resistance as well as the flowability of such compositions. The document is silent about the adhesion behavior of such a polyamide composition in combination with metals.
JP 2016-108531 refers to fluorene compounds to improve the bonding between reinforcing fibers and thermoplastic polymer matrices. The examples show that the modulus and tensile strength of the polymer compositions on base of polyphenylensulfide and carbon fibers can be improved by using bis(phenoxyethanol)fluorene.
In case of structural parts, in particular supporting parts, movable parts and/or security- or safety-relevant parts of a vehicle, an item of equipment or any other device demands on strength and reliability of the connection of metal and plastics have to be made which cannot be met by the prior art. However, a still not-solved drawback of fiber reinforced polyamide moulding compositions mentioned above is that the adhesion to metals is not sufficient.